Don't know about anyone else, but those three words add up to the epitome of Oxymorons in my world.

I’ve been noticing the cost of Li gradually creeping down, and over the long haul of life cycles may even pencil out to be cheaper than their wet cell alternatives...For the most part (e.g. excepting freezing temps) Li performance is quite remarkable...

MrWizard wrote:There is only one way to know SOC with sealed AGM batteries
And that is
Use an Amp Hour counter

The voltage vs SOC graphs for my particular AGMs do have a "bandwidth" or "range" so it is not very useful as a marker for SOC, as such. It seems to come out close enough cross-checking with AH count if I use the middle of the band.

Full resting is easy to establish with the voltmeter. Mine is 13.0v, which happens to be in the middle of the band. The middle for 50% is 12.4v, and that seems to correspond with the AH count using my estimated AH capacity as adjusted for temperature.

But it is true the volts vs SOC is something of a guess with that range of values. Of course the true capacity of the bank that is used for zeroing the AH counter is also an estimate.

I have always used the same voltage vs SOC at all ambient temperatures. Is that proper?

The AGM loses 15% capacity at freezing vs at rating temp of 77F, so is it still 13.0v when full? BUT at freezing temp, your charging rate of 14.8 is now supposed to be adjusted upwards to 15.2 volts to compensate for the cold. Voltage going opposite directions there maybe.

Does that matter for voltage vs SOC? The 100% full SOC is at the existing capacity, so "full" is a variable (with temperature changes). So is "full" 13.0 volts no matter what the real full capacity?

--When doing my capacity testing with Wets at the 20 hr rate, where I run the test for up to ten hours, checking SG, I am looking for the 50% "mark" according to the voltage/SG vs SOC table for the battery. Eg 12.1 volts is Trojan's 50%. Some batts use 12.2. My AGMs are more like 12.4 at 50%.

Say it gets down to that SG in 9 hours, I stop, wait for voltage bounce back for an hour (it only goes up a little more if I wait a day) to confirm the 50% voltage "matches" what the SG says, and that's my 50%. I can now say that using the time of 9 hours, the battery is at 90% of rated as new capacity.

I also use the AH counter to compare the count with the 50% mark using SG. It all comes out "right" by cross checking the SG, resting voltage, AH count, and temperature adjusted capacity.

But as Mr Wiz says, with AGMs you have no SG, so you can only go by the AH count from your estimated capacity (which is the thing you are trying to measure !!!)

What I do there is to go to say 8 hours, wait for bounce back and if it goes higher than the 50% value (12.4v for my AGMs --sort of anyway)then I restart the draw and check again at 9 hours. I check more often near the end.

So it all works the same but now can only cross check AH count with voltage after bounce back. Still, it gives you the idea of where your batts are compared with the last time you did it doing the test the same way. (Doing it the same way each time is important. Even if the capacity you get is "wrong", it is how much lower than last time that matters to you)

The glitch in the whole test whether AGM or Wets, is what 20 hr rate do you use? You don't know the capacity (yet). If you use the "as new" capacity for the rate, and the capacity is now less than that for your older battery, do you use a slightly lower amp draw? How much lower? I use the as new rate, which will give me a lower capacity measurement for the test result (Peukert)

I figure my test result at some lower capacity than it really is, will be on the good side for any testing inaccuracies, and by doing it the same way each test, I can tell how the batteries are doing. I run this test once a year to keep track.

BTW it is a problem to run the constant amps 20hr rate for 10 hours using my equipment. Using enough 12v lamps to hit the rate initially will leave you with fewer amps as the test goes on. Lamps dim with lower DC voltage and draw fewer amps. So you have to keep turning on more lights.

Inverter draw using some steady 120v draw is also a problem. The inverter will pull more amps as its DC voltage input declines, so now your initial amps will get too high.

Either way, you can anticipate that amps movement and average it out by starting a bit too high or low and it ends up lower or higher but averages out to the 20 hr rate, sort of. It comes out close enough--as long as you do it the same way as last time that's all you can do with the equipment you have for testing.

--You cannot use the AH counter by zeroing it at the start of the test and watch the count, because you don't know yet what the count will be when down to 50%. You have to use the bounce back voltage (and SG if wets) to say when you are at 50%, then you can just double your AH count to get the capacity (and compare that with the time it took out of 10 hours as a cross check)

MrWizard wrote:I think I would use the inverter method, I would go by watts not amps, the inverter will be better at maintain a constant power use, volts go down amps go up, still the same about of power being used

The inverter method offers that, but OTOH, you can find the right amount of amps better with different combos of lamps and fans to fine tune the amount for various battery 20 hr rates.

Takes a bit of experimenting with how many lights to get the right amount of amps, but each lamp singles and doubles, fluorescent or incand. gives you finer adjustments. (watching the amps on the Trimetric)

Not sure how to find an adjustable 120v load for the inverter. I did it with 120v lamps using different wattage bulbs, but not so good for fine tuning the amps.

I am useless with watts, so I go with amp hours that I can sort of understand. What is the 20hr rate of AH capacity in watts anyway? Too confusing for me!